“Gas filter correlation radiometry” (GFCR) is an optical remote sensing method used to produce highly sensitive measurements of “targeted” gases. Conventional GFCR measurement systems have made use of multiple single-element detectors where each detector measures light energy passed through a gas cell of the GFCR system. For multi-path systems designed for spectral measurement simultaneity, the light energy is split (prior to impingement on the gas cells) into a number of optical paths commensurate with the number of gas cells in the GFCR system. Such GFCR systems can use “back-end” electrical components that include balancing electronics coupled to the outputs of the detectors. In practice, the detector signals are electronically or mathematically balanced to be approximately equal when viewing light from an unattenuated light source such as the sun observed above the atmosphere from a satellite. That is, the multiple detectors' signals are differenced and balanced to give nearly zero difference during solar observation above the atmosphere. The key to making these measurements is the ability to determine the balance and rate of change of the difference signal before the observation in order to mitigate error due to drifts in detector response. To achieve the desired measurement accuracy of 1 part in 104 or greater, the balance must be known to 10−4 of the full broadband signal. Thus, small drifts in detector response, if not detected and corrected, can severely corrupt the difference measurement. However, because the conventional multiple-detector system/method requires continuous high-precision calibration of the balance condition (i.e. calibration of the signal drift due to changes in system response), many researchers abandoned the multiple-detector system/method in favor of single-detector, single-beam systems/methods that modulate the cell condition in some fashion.
Single-detector systems/methods can nearly eliminate detector instability as an error source by measuring both signals with the same detector. However, single-detector systems/methods present other technical hurdles depending on method of implementation. In one such single-detector system/method, the gas cell condition is modulated by changing pressure or optical mass thereby causing a significant decrease in sensitivity because the cell modulation produces a relatively small spectral difference between optical paths associated with the different gas cells. The signals are also difficult to model because of gas heating and cell state variation that may not reach uniform equilibrium. In addition, vibration and subtle optical changes can be problems for implementation requiring a steady field-of-view.
In another type of single-detector method/system, the light path is switched between a gas-cell path and a non-gas-cell (e.g., vacuum) path by either rapidly re-routing the beam (e.g., polarization switching techniques) or moving the gas cell into and out of the beam. However, both of these approaches introduce noise due to beam steering and loss of signal integration time due to time between modulated states, as well as subtle spectral response, polarization and field-of-view differences.
An even greater problem with any single-detector method/system is the loss of measurement simultaneity and/or the ability to exactly match field-of-views for gas and vacuum paths. That is, if the scene changes during the time necessary to switch between modulated states or because of field-of-view mismatch, the change in normalized difference signal (caused by scene brightness variation) will corrupt the data interpretation that assumes the difference signal is produced solely by spectral variation. For example, a satellite traveling at 7 km/sec encountering a 1% per kilometer change in mean scattering brightness over the field-of-view will experience a fractional brightness change of 10−4 in 1.4 milliseconds which could be falsely interpreted as spectral variation. This presents a severe problem for the single-detector method, or any method that does not make simultaneous and spatially identical measurements of the two states (i.e., gas path and vacuum path).
More recently, another single-detector system/method attempted to remedy the above-described problems by utilizing a two-beam approach that splits the light (beam) energy and forms two pupil images on the same two-dimensional detector array. See U.S. Pat. No. 7,460,235. While this patented approach solved the requirement of temporal simultaneity, some applications incurred small but unacceptable degrees of beam mismatch in terms of field-of-view, spectral response, polarization, and signal linearity.